There are many applications in positioning systems which utilize linear motors. These linear motors produce linear forces in response to an electrical current input and are desirable for providing linear forces accurately with high-precision and fast response times.
For example, U.S. Pat. No. 4,749,921 entitled “Linear Motor with Non-Magnetic Armature” discloses a design for a linear motor that incorporates an armature assembly having a plurality of coil windings in a three-phase linear motor system comprising three coil sets. The coil sets cooperate with a plurality of permanent magnets to produce forces in a linear direction through electromagnetic interaction when current is supplied through the coils. Such linear motors are capable of producing specific rated forces depending upon the current applied to them. In order to produce different driving forces, the amount of current supplied to the coils must be varied accordingly.
FIG. 1 is an isometric view of a gantry system 10 that is operable to position an object 12 along an X-Y plane. The gantry system 10 generally comprises a base support 14 which includes vertical sidewalls, a gantry beam 16 having separate sections 18a, 18b and supported at its respective ends by each vertical sidewall, a linear guide rail 20 for guiding movement of one section 18a of the gantry beam 16 along one vertical sidewall, and a bearing such as an air bearing 22 for supporting a second section 18b of the gantry beam 16 along the other vertical sidewall.
There is a pair of linear motors 24, 24′ each driving a respective end of the first section 18a and the second section 18b substantially synchronously in parallel directions to position the gantry beam 16 along a linear axis (ie. the Y axis in FIG. 1). Each linear motor 24, 24′ comprises a magnetic assembly including rows of magnets cooperating with a coil bracket including coil windings which is movable relative to the magnetic assembly. The magnet assemblies may be mounted to the base support 14 whereas the coil brackets may be mounted to the gantry beam 16.
The object 12 is slidably mounted on the gantry beam 16 and is movable along an axis parallel to the length of the gantry beam 16 (ie. the X axis in FIG. 1). The object is preferably drivable by another linear motor. For convenience, the axis along which the object 12 is slidably movable is referred to as the X axis and the axis along which the guiding rail 20 guides the gantry beam 16 is referred to as the Y axis. It would be appreciated that a combination of movement of the object 12 in the X and Y axes driven by the respective linear motors serve to move the object 12 to various locations on an X-Y plane.
Flexibility in the gantry beam 16 is introduced by incorporating a flexure 26 which separates the two sections of the gantry beam 14. The flexure 26 allows the second section 18b to be deflectable relative to the first section 18a of the gantry beam 16 about the flexure 26.
During operation, it is preferable that the two ends of the gantry beam 16 are driven simultaneously by the same distance so that the beam is maintained parallel to the X axis. However, if there is asynchrony between the linear motors 24, 24′, an end driven by one linear motor may be driven further than another end driven by the other linear motor.
With the flexure 26 incorporated into the gantry beam 16, one section of the beam 16 is allowed to deflect and rotate due to the limited degree of movement of an end of the gantry beam 16 supported on the air bearing. The other section of the gantry beam 16 remains relatively fixed. Thus, the stresses on the system 10 can be reduced, and the rotational resonant frequency is reduced to a value that is no longer difficult to control.
Since the system 10 encompasses a flexure 26 that allows one axis of the gantry 16 to rotate, it is necessary to provide a compensatory or correction force to maintain the parallelism of the gantry beam 16 in the X axis. Thus, the two linear motors 24, 24′ should be operable for both linear motion in the Y axis and for relative motion in the Y direction to reposition the gantry beam 16 about the Z axis. However, depending upon the payload of the system 10, it is likely that the control gains of the two linear motors 24, 24′ will not be identical, and will, perhaps greatly differ. With these two control axes (linear and rotational) having large gain variations and yet being controlled by the same linear motor, this is likely to lead to a significant control mismatch and noise, reducing the performance of the gantry system 10.
It would be desirable to be able to drive the gantry beam 16 along a control axis with a higher inertia with a linear motor with a larger motor force constant, and along a control axis with a lower inertia with a linear motor with a lower motor force constant. It would also be desirable for both of these linear motors to be designed in a structure that is compact.